This invention relates to oven brazing in general, and specifically to a method for improving the brazing quality of a heat exchanger or the like that has a significant mass asymmetry.
Braze ovens used to manufacture aluminum heat exchangers can be roughly divided into the radiant and convective type, distinguished by the heating method involved. Radiant braze ovens have an elongated, double walled tubular muffle, heated by a radiant heat source, typically an electric resistance wire. The heat exchanger workpiece is run through the muffle, and heated to braze temperature, around 1100 to 1200 Degrees F., by radiant energy. In a convective braze oven, heated air is blown over the workpiece to heat it. Each technique has its own unique advantages and drawbacks.
A potential problem with either brazing method involves mass differentials unique to the heat exchanger itself. In most cases, the mass differential is a symmetrical one. That is, a relatively low mass central core, comprising thin tubes and thin corrugated fins, is flanked on each side by heavier, but equally massive, manifold tanks. The tanks will heat up to braze temperature more slowly than the relatively less massive core. At least in the case of convective brazing, a known solution is to preferentially direct a greater volume or flow rate of heated air at and over the more massive side tanks, as disclosed in U.S. Pat. No. 5,322,209. This, of course, is inapplicable in the case of a radiant oven.
In the case of a radiant oven, the mass differential issue is exacerbated in a situation where the mass differential is also asymmetric. For example, a heat exchanger such as a condenser may have one manifold tank on one side that is considerably more massive than the tank on the opposite side, as when a receiver dehydrator tank is integrated into it. Since radiant heating rate is a function of how readily the surface of the part absorbs radiation, one known technique to compensate for the mass differential is to create a counterbalancing thermal absorption differential. For example, if each of the different mass tanks has a relatively shiny surface, the surface of the more massive tank may be dulled somehow, made more absorbent to a compensating degree. Black paint is one obvious approach, but it would be difficult to find a paint that would survive the temperatures involved. Another known technique is to shot blast the surface of the more massive tank, or acid etch it, thereby dulling and roughening it. These are both environmentally undesirable, expensive, and difficult to control or tailor to differing conditions.
The subject invention provides a novel method of varying surface thermal absorption rate which, rather than removing material from the surface, or painting it, modifies the surface absorbtivity by adding a layer of temperature resistant material that varies the surface roughness sufficiently to create the desired absorbtivity differential.
In the embodiment disclosed, a high temperature material, such as steel metal powder, is flame sprayed onto the surface of the more massive areas of the workpiece. The powder grain size and other spray parameters are chosen to give a layer of sufficiently increased surface roughness to in turn increase the surface absorbtivity sufficiently to compensate for the mass differential. Heating within the braze oven is more even and effective.